Method for cyclically preparing monomer boron and coproducing potassium cryolite using potassium fluoborate as intermediate material

ABSTRACT

A method for cyclically preparing monomer boron and coproducing potassium cryolite using potassium fluoborate as an intermediate material, which includes following steps: A) adding hydrofluoric acid to boric acid or boron oxide to enable a reaction to form fluoboric acid; B) adding a potassium sulphate aqueous solution to the fluoboric acid to enable a reaction to form the potassium fluoborate; C) putting the potassium fluoborate into a reactor, adding aluminium to react with the potassium fluoborate to form the monomer boron and potassium cryolite; D) extracting the potassium cryolite, sending the potassium cryolite to a rotary reaction kettle together with concentrated sulphuric acid to enable a reaction to form hydrogen fluoride gas and aluminium potassium sulphate, potassium sulphate, collecting the hydrogen fluoride gas and dissolving it into water to obtain the hydrofluoric acid; E) recycling the obtained hydrofluoric acid to Step A to leach the boric acid or boron oxide.

TECHNICAL FIELD OF THE INVENTION

The disclosure relates to a method for preparing monomer boron, and inparticular to a method for cyclically preparing the monomer boron andcoproducing low-molecular-ratio potassium cryolite using potassiumfluoborate as an intermediate material.

BACKGROUND OF THE INVENTION

Generally, there are two methods for producing element boron inindustry.

(1) Magnesium reduction process, in which boric acid and magnesiumpowder are mainly taken as raw materials, the boric acid for industrialuse is put in a stainless steel plate and then the stainless steel plateis put in a tube furnace to evenly heat to a temperature of 250 DEG C.under a pressure-reducing condition, so that the boric acid dehydratesto form boron oxide: 2H₃BO₃=B₂O₃+3H₂O; then the boron oxide is crushedto 80 mesh and is fully mixed with the magnesium powder at the ratio of3:1 (mass ratio); and then the mixture is put in a reaction tube toperform a reduction reaction at a temperature of between 850 and 900 DEGC. in the tube furnace under vacuum, so that the boron oxide is reducedto the element boron, wherein this reaction is a thermal reaction whichcan be finished quickly: B₂O₃+3Mg=3MgO+2B; the materials obtained afterthe reaction is finished are dipped in water for two days and then areboiled for 4 hours in hydrochloric acid so as to be free of impuritiessuch as magnesium oxide, then the materials are washed off acid bywater; in order to remove the impurities, it is necessary to repeat acidpickling and water washing for one time in the same condition, thus,boron powder with boron content of about 85% is obtained; in order toimprove the quality of boron and to further remove magnesium, the boronpowder above can be added to the boron oxide which is ten times the massof the boron powder to be evenly mixed and heated to a temperature ofbetween 800 and 850 DEG C. in the reaction furnace under vacuum and keptfor 3 to 4 hours; then the material is taken out and washed off boronoxide by water; after processes of acid pickling and water washingagain, the material is filtered and dried, thus, boron powder with boroncontent of over 90% is obtained.

(2) Aluminium reduction process, in which industrial borax is generallytaken as a raw material and is put in a melting furnace of over 750 DEGC. at normal pressure to dehydrate 10 crystal water to form anhydridesodium tetraborate; after being cooled, coarse-crushed and fine-crushed,the anhydride sodium tetraborate is well mixed with sulphur andaluminium powder at a certain ratio, then the mixture is put in acast-iron reaction furnace to perform a reaction at a high temperature:Na₂B₄O₇+4Al=4B+Na₂Al₂O₄+Al₂O₃; after being cooled, frits are taken outof the reaction furnace and are crushed first, then the crushed fritsare dipped in hydrochloric acid and then in hydrofluoric acid, next, thecrushed frits are washed by water and alkali (5 mass percent NaOHsolution), finally, the crushed frits are wasted by water, separated anddried to obtain element boron.

The two methods above mainly have disadvantages of low yield rate andhigh preparation cost, and the content of the product obtained is lessthan 90%.

The method for preparing potassium fluoroaluminate (potassium cryolite)in industry generally adopts a synthesis method, in which anhydroushydrofluoric acid reacts with aluminium hydroxide to form fluoaluminicacid; then the fluoaluminic acid reacts with potassium hydroxide at ahigh temperature; after processes of filtering, drying, melting andcrushing, the potassium fluoroaluminate is prepared, wherein thereaction formula is as follows: 6HF+Al(OH)₃=AlF₃.3HF+3H₂O,AlF₃.3HF+3KOH=K₃AlF₆+3H₂O; the potassium cryolite synthesized by thismethod has a relative molecular weight of 258.28, with a molecularformula of AlF₃.mKF (m=3.0) and a melting point of between 560 and 580DEG C.; the potassium cryolite synthesized by the industrial synthesismethod generally has a molecular ratio of m=2.0-3.0, and it is difficultto obtain the relatively pure potassium cryolite of a low molecularweight with a molecular ratio of m=1.0-1.5.

SUMMARY OF THE INVENTION

In order to solve the problem how to prepare relatively pure potassiumcryolite with a low molecular ratio massively and industrially, so as tomeet the requirement of the low-temperature aluminium electrolysisindustry on electrolyte, the inventor has done a great deal of researchin the selection of intermediate material and the circular process ofcoproduct and unexpectedly finds that the potassium cryolite of a lowmolecular weight with a molecular ratio of m=1.0 can be produced whilemonomer boron is produced by a thermo-chemical sulphate reductionprocess with potassium fluoborate as an intermediate material; moreover,if a chemical reaction is further performed on the potassium cryolite,the monomer boron can be cyclically produced. The disclosure has asimple process; compared with the conventional popular preparationmethod, the disclosure achieves a lower cost; the product obtained is ofhigh quality and can be cyclically used, thus the product efficiency isimproved and the pollution to environment is reduced.

The disclosure provides a method for cyclically preparing monomer boronand coproducing potassium cryolite using potassium fluoborate as anintermediate material, which includes the following steps:

A) adding hydrofluoric acid to boric acid or boron oxide to enable areaction at a temperature of between 100 and 200 DEG C. to formfluoboric acid, wherein the reaction formula involved is:H₃BO₃+4HF=HBF₄+3H₂O, B₂O₃+8H=2HBF₄+3H₂O;

B) adding a potassium sulphate aqueous solution to the fluoboric acid toenable a reaction to form a potassium fluoborate precipitate,centrifuging and rinsing the potassium fluoborate precipitate to obtainthe intermediate material potassium fluoborate, wherein the reactionformula involved is:2HBF₄+K₂SO₄=2KBF₄↓+H₂SO₄;

C) putting dried potassium fluoborate into a reactor, injecting an inertgas to the reactor after vacuumizing, heating the reactor to atemperature of between 700 and 800 DEG C., adding aluminium in thereactor and stirring quickly to enable a reaction for 4 to 6 hours toform monomer boron and potassium cryolite, or, putting the aluminiuminto the reactor, injecting an inert gas to the reactor aftervacuumizing, heating the reactor to the temperature of between 700 and800 DEG C., adding dried and flowable potassium fluoborate in thereactor and stirring quickly to enable a reaction for 4 to 6 hours toform the monomer boron and potassium cryolite, wherein the reactionformula involved is: KBF₄+Al=B+AlF₃.KF;

D) extracting molten liquid potassium cryolite; after the molten liquidpotassium cryolite is cooled, crushing and sending it to a rotaryreaction kettle quantificationally together with concentrated sulphuricacid to enable a reaction at a temperature of between 400 and 500 DEG C.to form hydrogen fluoride gas and aluminium potassium sulphate,potassium sulphate; collecting the hydrogen fluoride gas and dissolvingit into water to obtain a hydrofluoric acid aqueous solution; reactingthe mixture of the aluminium potassium sulphate and the potassiumsulphate with an aqueous solution of potassium hydroxide after crushingthe mixture of the aluminium potassium sulphate and the potassiumsulphate, and obtaining the aqueous solution of potassium sulphate afterseparating out the solid aluminium hydroxide, wherein the reactionformula involved is: AlF₃.KF+2H₂SO₄=4HF↑+KAl(SO₄)₂,KAl(SO₄)₂3KOH=2K₂SO₄+Al(OH)₃↓;

E) recycling the obtained hydrofluoric acid aqueous solution and theaqueous solution of potassium sulphate to the front end to leach theboric acid or boron oxide, so as to achieve the purpose of cyclicallypreparing the intermediate material potassium fluoborate.

The technical scheme above achieves the purposes as follows: thedisclosure provides a method for preparing a low-temperature aluminiumelectrolyte (potassium cryolite) with a low molecular ratio, of whichthe molecular weight is 142, and provides a method for cyclicallyextracting and using the element boron in the potassium cryolite so asto protect underground fluorite resources, wherein the coproductpotassium cryolite has a good application prospect and can be recycled;compared with the conventional method for preparing monomer boron, thedisclosure has a simple and short process, a low comprehensive cost ofproduction, a high production efficiency and a low pollution toenvironment.

As a further improvement of the disclosure, in Step C, the aluminium ismolten aluminium which is added in the reactor in a dripping way, or thealuminium is added in the reactor first and then dried potassiumfluoborate is batch-fed in the reactor after the aluminium is molten,wherein the completeness of this reaction can be greater than or equalto 95%.

As a further improvement of the disclosure, in Step C, the inert gas isargon.

Compared with the conventional art, the disclosure achieves advantagesas follows: the disclosure provides a method for preparing potassiumcryolite of a low molecular weight with a low molecular ratio of m=1.0,which is suitable for massive industrial production, so as to meet therequirement of the low-temperature aluminium electrolysis industry onelectrolyte; the disclosure produces the monomer boron simultaneously,and compared with the conventional art has a simple preparation process,a high product yield and has the boron content greater than 90%;moreover, the boron can be used as an intermediate material forproducing various borides and boron metal alloys. Meanwhile, since theelement fluorine in the potassium cryolite can be cyclically extracted,the life of fluorite resources can be prolonged and the pollution toenvironment is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process route chart of preparing monomer boron andpotassium cryolite according to the disclosure; and

FIG. 2 shows a process flowchart of preparing monomer boron andpotassium cryolite according to the disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosure is described below in further detail through specificembodiments.

Embodiment 1

Putting 0.62 tons of boric acid and 0.35 tons of boron oxide into areaction kettle; adding 4 tons of 20% (mass percent) hydrofluoric acidin the reaction kettle to react with the boric acid or boron oxide at atemperature of 100 DEG C. to form fluoboric acid; adding 3 tons of 30%(mass percent) potassium sulphate aqueous solution to the fluoboric acidto enable a reaction to form a potassium fluoborate precipitate;centrifuging, rinsing and drying the potassium fluoborate precipitate toobtain 1.12 tons of intermediate material potassium fluoborate; puttingthe 1.12 tons of dried potassium fluoborate into another reactor,injecting an inert gas to the reactor after vacuumizing, heating thereactor to a temperature of 700 DEG C., dripping molten aluminium intothe reactor slowly in accordance with a reaction ratio and stirringquickly, wherein the materials are completely reacted after 5 hours andmonomer boron and potassium cryolite are generated; extracting moltenliquid potassium cryolite; crushing the molten liquid potassium cryoliteafter it is cooled, weighing and sending it to a rotary reaction kettlequantitatively together with concentrated sulphuric acid which is addedin accordance with a reaction ratio, reacting the potassium cryolitewith the concentrated sulphuric acid at a temperature of between 400 and500 DEG C. to form hydrogen fluoride gas and aluminium potassiumsulphate, potassium sulphate; collecting the hydrogen fluoride gas anddissolving it into water to obtain a hydrofluoric acid aqueous solution;reacting the aluminium potassium sulphate, the potassium sulphate withan aqueous solution of potassium hydroxide after crushing the aluminiumpotassium sulphate and the potassium sulphate, and obtaining thepotassium sulphate aqueous solution after separating out the solidaluminium hydroxide, wherein the obtained hydrofluoric acid aqueoussolution and the potassium sulphate aqueous solution can be recycled tothe front end to leach the boric acid or boron oxide, so as to achievethe purpose of cyclically preparing the intermediate material potassiumfluoborate.

Embodiment 2

Putting 0.62 tons of boric acid and 0.35 tons of boron oxide into areaction kettle; adding 4 tons of 20% (mass percent) hydrofluoric acidin the reaction kettle to react with the boric acid or boron oxide at atemperature of 100 DEG C. to form fluoboric acid; adding 3 tons of 30%(mass percent) potassium sulphate aqueous solution to the fluoboric acidto enable a reaction to form a potassium fluoborate precipitate;centrifuging, rinsing and drying the potassium fluoborate precipitate toobtain 1.12 tons of intermediate material potassium fluoborate; weighingaluminium in accordance with a reaction ratio and putting the aluminiuminto another reactor, injecting an inert gas to the reactor aftervacuumizing, heating the reactor to a temperature of 700 DEG C., addingthe 1.12 tons of dried and flowable potassium fluoborate in the reactorin a measurable flowing way and stirring quickly, wherein the materialsare completely reacted after 5 hours and monomer boron and potassiumcryolite are generated; extracting molten liquid potassium cryolite;crushing the molten liquid potassium cryolite after it is cooled,weighing and sending it to a rotary reaction kettle quantitativelytogether with concentrated sulphuric acid which is added in accordancewith a reaction ratio, reacting the potassium cryolite with theconcentrated sulphuric acid at a temperature of between 400 and 500 DEGC. to form hydrogen fluoride gas and aluminium potassium sulphate,potassium sulphate; collecting the hydrogen fluoride gas and dissolvingit into water to obtain hydrofluoric acid; reacting the mixture of thealuminium potassium sulphate and the potassium sulphate with an aqueoussolution of potassium hydroxide after crushing the mixture of thealuminium potassium sulphate and the potassium sulphate, and obtainingthe potassium sulphate aqueous solution after separating out the solidaluminium hydroxide, wherein the obtained hydrofluoric acid aqueoussolution and the potassium sulphate aqueous solution can be recycled tothe front end to leach the boric acid or boron oxide, so as to achievethe purpose of cyclically preparing the intermediate material potassiumfluoborate.

The above are the further detailed description of the disclosure made inconjunction with specific preferred embodiments; it can not beconsidered that the specific embodiment of the disclosure is onlylimited to the description above. For the common technicians in thetechnical field of the disclosure, umpty simple deductions orsubstitutes can be made without departing from the concept of thedisclosure and they are deemed to be included within the scope ofprotection of the disclosure.

What is claimed is:
 1. A method for cyclically preparing monomer boronand coproducing potassium cryolite using potassium fluoborate as anintermediate material, including the following steps: A) addinghydrofluoric acid to boric acid or boron oxide to enable a reaction at atemperature of between 100 and 200 DEG C. to form fluoboric acid; B)adding a potassium sulphate aqueous solution to the fluoboric acid toenable a reaction to form a potassium fluoborate precipitate,centrifuging and rinsing the potassium fluoborate precipitate to obtainthe intermediate material potassium fluoborate; C) putting driedpotassium fluoborate into a reactor, injecting an inert gas to thereactor after vacuumizing, heating the reactor to a temperature ofbetween 700 and 800 DEG C., adding aluminium in the reactor and stirringquickly to enable a reaction for 4 to 6 hours to form the monomer boronand potassium cryolite, or, putting the aluminium into the reactor,injecting an inert gas to the reactor after vacuumizing, heating thereactor to the temperature of between 700 and 800 DEG C., adding driedand flowable potassium fluoborate in the reactor and stirring quickly toenable a reaction for 4 to 6 hours to form the monomer boron andpotassium cryolite; D) extracting molten liquid potassium cryolite;after the molten liquid potassium cryolite is cooled, crushing andsending it to a rotary reaction kettle quantificationally together withconcentrated sulphuric acid to enable a reaction at a temperature ofbetween 400 and 500 DEG C. to form hydrogen fluoride gas and aluminiumpotassium sulphate, potassium sulphate; collecting the hydrogen fluoridegas and dissolving it into water to obtain a hydrofluoric acid aqueoussolution; reacting the aluminium potassium sulphate with an aqueoussolution of potassium hydroxide after crushing the aluminium potassiumsulphate, and obtaining the aqueous solution of potassium sulphate afterseparating out the solid aluminium hydroxide; and E) recycling theobtained hydrofluoric acid aqueous solution and the aqueous solution ofpotassium sulphate to the front end to leach the boric acid or boronoxide, so as to achieve the purpose of cyclically preparing theintermediate material potassium fluoborate.
 2. The method for cyclicallypreparing monomer boron and coproducing potassium cryolite usingpotassium fluoborate as an intermediate material according to claim 1,wherein in Step C, the aluminium is molten aluminium which is added inthe reactor in a dripping way or the dried and flowable potassiumfluoborate is added in the reactor in a measurable flowing way.
 3. Themethod for cyclically preparing monomer boron and coproducing potassiumcryolite using potassium fluoborate as an intermediate materialaccording to claim 1, wherein in Step C, the inert gas is argon.